The diurnal variation characteristics of summer precipitation in Hubei Province are highly remarkable. The diurnal variation curve of precipitation is of double - peak structure, showing characteristics of semi-diurnal circulation. The peaks of precipitation occur at 0800 and 1700 BST respectively, and the precipitation levels in the two time periods are the same. The valley values occur at 12:00 and 23:00BST. The diurnal variation curves of both precipitation frequency and intensity are of double - peak structure, with one weaker than the other. This is mainly related to eastward-moving weather systems from the Tibet Plateau and the effects of local thermal forcing. The precipitation intensity during 1500 to 1800 BST shows an obvious interannual increasing trend for the past 14 years.
Regionally, the diurnal variation characteristics of precipitation in Hubei Province are noticeably distinct, with the precipitation peak or valley values in different regions occurring at different times. The primary or secondary peak of precipitation amount occurs in the evening or at midnight in Northwest Hubei; however, whilst this is also cause in Southwest Hubei, the peak amount is not subdivided into a primary or secondary value. Also, the valley values of precipitation in both areas occur at midday. Meanwhile, the primary or secondary peak of precipitation amount occurs in the morning or in the evening in Jianghan Plain, and the valley value of precipitation occurs at midnight. The situations in Northeast Hubei is similar to that of Jianghan Plain. The diurnal variation curve in Southeast Hubei shows a smooth double - peak structure, and the primary or secondary peak value occurs in the evening or in the morning, which is opposite to that in Northeast Hubei and Jianghan Plain. The valley value of precipitation occurs at midnight. The diurnal variation curve of precipitation frequency and precipitation intensity is similar to that of precipitation amount.
The most active 0-20mm·h-1 precipitation region is the western mountain area of Hubei Province. The short-duration heavy precipitation frequency in Hefeng and Xuan’en is over 13%, while the precipitation frequency in eastern Plain and hills is lower. The precipitation frequency in Jianghan Plain and Northeast Hubei is about 8%, and in Southeast Hubei is about 10%. The trend of 0-20mm· h-1 precipitation is a decreasing one (stations with a decreasing trend account for 67.53% of the total), and the precipitation frequency increases mainly in Western and Southeast Hubei. Opposite to the frequency distribution of 0-20mm·h-1 precipitation, the most active regions with precipitation frequencies of >= 20mm· h-1 and >= 50mm· h-1 are mainly in Jianghan Plain and Eastern Hubei. In general, the frequency of short- duration heavy precipitation is larger in the east than in the west, and larger in the plain than in mountain areas. Those stations with an increasing trend of short duration heavy precipitation account for 53.24% of total stations, increasing at 0-2.5% (10yr)-1. These stations are mainly distributed in Southwest Hubei, Jianghan Plain and Eastern Hubei. The diurnal variation curve of 0-20mm· h-1 precipitation is of double-peak structure, with one weaker than the other. The primary or secondary peak value occurs in the morning or in the evening. The precipitation frequency at 1200 and 2400 BST is the lowest. The peak value of short-duration heavy precipitation (>=20mm· h-1) occurs at 1700 BST, and the valley value occurs at 1200 BST. Short-duration catastrophic precipitation (>=50mm· h-1) occurs at 1500 – 2000 BST, and rarely occurs during 0300-1400 BST.
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